1. Field of the Invention
The present invention relates to imaging lenses which form an image of an object on a solid-state image sensor such as a CCD sensor or a C-MOS sensor used in a compact image pickup device, and more particularly to imaging lenses which are built in image pickup devices mounted in increasingly compact and low-profile smart phones and mobile phones, PDAs (Personal Digital Assistants), game consoles, information terminals such as PCs, and home appliances with a camera function.
2. Description of the Related Art
In recent years, there has been a general tendency that many information terminals have a camera function. Also, home appliances with a camera function have been introduced into the market. For example, a user who is away from home can see in real time what is going on at home, through the camera mounted in a home appliance by telecommunication between the home appliance and his/her smart phone. It is thought that products which enhance consumer convenience by adding a camera function to an information terminal or home appliance will be increasingly developed in the future. The camera mounted in such a product is expected not only to provide high resolution to cope with an increase in the number of pixels but also to be compact and low-profile and offer high brightness and a wide field of view. Particularly, the imaging lens to be built in a mobile terminal is strongly expected not only to be low-profile enough to be applicable to a low-profile product but also to deliver high imaging performance.
However, in order to provide a low-profile imaging lens with a wide field of view and high brightness as described above, the following problem has to be addressed: it is difficult to correct aberrations in the peripheral area of the image and ensure high imaging performance throughout the image.
In the related art, for example, the imaging lens described in JP-A-2011-085733 (Patent Document 1) is known as a compact high-resolution imaging lens.
Patent Document 1 discloses an imaging lens which includes, in order from an object side, a first lens group including a first lens having a convex surface on the object side, a second lens group including a second lens having a concave surface on an image side, a third lens group including a meniscus third lens having a concave surface on the object side, a fourth lens group including a meniscus fourth lens having a concave surface on the object side, and a fifth lens group including a meniscus fifth lens having an aspheric surface with an inflection point on the object side. This configuration is intended to provide a compact imaging lens system which offers high resolution.
The imaging lens described in Patent Document 1 has a total track length of about 6.0 mm and the ratio of total track length to the diagonal length of the effective imaging plane of the image sensor (hereinafter referred to as the “total length to diagonal ratio”) is about 0.9, offering a lens system which is relatively low-profile and corrects aberrations properly. However, its F-value is about 2.8, so its brightness is not sufficient. Furthermore, its field of view is about 65 degrees, which is insufficient to meet the demand for a wide field of view. Furthermore, in order for this configuration to offer an F-value of 2.5 or less and a field of view of 70 degrees or more, the problem with difficulty in correction of aberrations in the peripheral area of the image must be addressed.